Such a part is for example designed to form a floor covering carpet in a motor vehicle.
The carpets used in automobiles as floor coverings are essentially needlepunched mats of the “flat needlepunched” or “Dilour™” needlepunched type.
These mats belong to the nonwoven family. They are preferable to traditional woven coverings, because they are deformable and can hug the shapes of vehicle floors.
This “shaping” of floor mats to hug the configuration of vehicle floors occurs during the thermoforming operation. Since these mats are generally associated with sublayers (heavy mass or felts) to create an acoustic complex allowing sound insulation or absorption, thermoforming takes place at the softening temperature of those sublayers (for example, beginning at 110° C. if the sublayer is a polyolefin-based heavy mass).
The “Dilour™” needlepunched products are also called “velvet needlepunch”, since their surface appearance is similar to that of velvet. This “velvet” in the case at hand is made up of randomly distributed fibers. These fibers may be in the form of loops or individual fibers (trimmed loops).
These mats are formed from a web of fibers (called precursor web and having undergone a first needlepunching, called pre-needlepunching), for example on a Dilour™ machine made up of a conveyor equipped with a set of brushes and a needlepunching head equipped with needle boards (also called combs) of the “crown” type optionally completed by “fork” needles.
Crown-type needles, which generally have a triangular section, have a single barb per edge positioned just in front of the tip of the needle at a same distance from the tip. These barbs are thus positioned to drive the fibers while forming loops of equal lengths. Furthermore, the barbs being situated on the side, they can grasp the fibers in the entire thickness of the precursor web. This type of needle therefore allows a maximum velvet output.
Nevertheless, because the barbs are situated on the edges, they are shallow and the number of fibers driven per movement is small. To form the velvet, a large number of penetrations are therefore required, i.e., a high needlepunching density, which is detrimental to productivity.
Fork needles have a circular section and have no tip, but an end in the shape of a fork (upside down U). They can only drive fibers situated essentially on the back of the precursor web (where they penetrate), but in larger number than crown needles, because the dimensions of the fork are much larger than those of a barb.
Nevertheless, these needles are much more aggressive (they break some of the fibers that they encounter). Using them in large number is therefore considered to be detrimental to the planar cohesion of the Dilour™ needlepunch, and therefore its ability to deform without tearing during thermoforming.
That is why it is preferred to combine these two types of needles to obtain both acceptable velvet output and productivity, without being too detrimental to the planar cohesion. Thus, there are for example combs made up of half crown needles and half fork needles.
These needles drive part of the fibers of the web inside the brushes of the conveyor, over a depth that will correspond to the height of the finished produced velvet.
Simultaneously with the establishment of the velvet, needlepunching makes the web denser by contributing to trimming the fibers in the part of the web remaining on the surface of the brushes (called “base”). In other words, the thickness of the base is reduced as the fibers mix together.
The height of the velvet generally varies between 2 mm and 5 mm, the thickness of the base varying from 1 mm to 3 mm, while the thickness of the precursor web is approximately 5 to 10 mm.
When the velvet is trimmed (in a step immediately following the “Dilour processing”, the height of the velvet loops being shaved), the fibers making up the velvet assume the shape of a U, the base of the U being located in the base. Hereinafter, it will be considered that the fibers of the velvet advantageously assume this form.
Next, the mats formed by needlepunching are consolidated by a resin that binds the fibers to each other in the base. In fact, the mechanical cohesion that they receive due to the needlepunching is insufficient to guarantee proper performance during use, once installed in the vehicle (in particular resistance to abrasion, filamentation, etc.).
These resins are typically latexes of the SBR (Styrene Butadiene Rubber) type and are applied on the backside of the mat in the form of an aqueous dispersion using known coating means, followed by expression to cause the dispersion to penetrate the base. The mats are next dried in furnaces to evacuate the water. The surface mass of the dry latex extract remaining in the base after drying represents between 15 and 30% of the surface density of the web.
Using latex has drawbacks, because the penetration of the dispersion inside the base is difficult to control (in no case may it pass through the base, which would pollute the fibers and the velvet), in part due to capillarity phenomena caused in the fibrous network, which is inherently random. Furthermore, latexes are thermohardened (or cross-linked) polymers that are difficult to recycle, and the latex residues must be stored, since they are potentially hazardous for the environment.
To offset these problems, it is known to use, in place of the latex, thermofusible fibers that are dispersed in the web of fibers and that are melted after needlepunching. These fibers can be single-component fibers (with a base of a same polymer) or bi-component fibers (for example of the body and core type, where the body is made up of a polymer having a melting temperature lower than that of the polymer making up the core). The polymer with a low melting point most generally used is polyethylene PE, the melting temperature of which varies from 90° C. to 120° C.
To obtain bonding similar to that of latex, it is necessary to introduce a large quantity of thermofusible fibers into the web, as described in EP 518,690. This increases the cost of the product (fusible fibers generally being more expensive than standard fibers). This introduction is also detrimental to its surface appearance, given the appearance of mottling on the velvet layer after thermoforming.
In fact, needlepunching in the Dilour™ machine indifferently drives the base fibers and the binder fibers, which are therefore found in equal proportions in the velvet and the precursor web. However, the thermoforming is done at a temperature close to the melting temperature of the binder fibers, such that the binder fibers present in the velvet can stick the base fibers of the velvet to each other and prevent them from straightening after crushing caused by closing the thermoforming mold.
In order to offset this problem, EP 2,286,012 describes a method in which a first web comprising thermofusible fibers is used to form the base and a second web with no thermofusible fibers is used to form the finished layer, the webs being assembled together during needlepunching of the second web.
The implementation of such a method requires a machine comprising two needlepunching heads positioned on a same conveyor, which requires a significant investment, and can complicate the implementation of the method.
EP 2,664,702 describes a method in which a mat is obtained by needlepunching, then consolidated using a latex layer. The mat comprises binder fibers with a high weight percentage, as indicated in the EP 518,690.
A layer of adhesive, for example made from a thermoplastic polymer, is adhered to the back of the mat, to allow bonding with a substrate (generally performing an acoustic function) or to allow improved rigidity of the mat that is essential to mount the mat in the vehicle. This layer does not penetrate the web of needlepunched fibers.